According to "Ullmann's Encyclopedia of Industrial Chemistry, Fifth Edition, Vol. A 7, VCH Verlagsgesellschaft mbH, Weinheim, 1986, pages 67-81", chromic acid is produced industrially by three different processes. Sodium dichromate is used as starting material for all these processes. In the process of preparation of sodium dichromate, chromium ore is roasted with sodium carbonate and/or sodium hydroxide and atmospheric oxygen. The clinker leaving the furnace used for this roasting is leached with water and/or a solution containing chromium. After adjustment of the pH of the mash to 7-9.5 with sulphuric acid and/or sodium dichromate solution, the sodium monochromate solution obtained is freed from insoluble constituents of chromium ore by filtration. The chromate-containing residue obtained must then be worked up by an expensive process.
For the preparation of sodium dichromate, the chromate ions of the solution are converted into dichromate ions by acidification with sulphuric acid and/or acidification with carbon dioxide under pressure. Chromate-containing sodium sulphate is inevitably formed in the process and must be worked up.
For the preparation of chromic acid by the so-called wet process, the sodium dichromate solution is concentrated and sulphuric acid is added to the solution so that chromic acid crystallizes.
In the so-called melt or dry process, sodium dichromate crystals are reacted with concentrated sulphuric acid in a molar ratio of approximately 1:2 at temperatures of about 200.degree. C. Common to both these processes is the inevitable formation of sodium bisulphate contaminated with chromate either as a melt or as an aqueous solution.
This disadvantage is avoided in the third process, which consists of the electrolysis of sodium dichromate in aqueous solution as described, for example, in DE-C 3 020 260 or CA-A 739 447.